Hot-gas engine and method of manufacturing same

ABSTRACT

A hot-gas engine, containing a working medium which consists mainly hydrogen. The walls of the working spaces of the engine which are at a higher temperature during operation are covered with a silicon nitride layer. The shape of the relevant wall portions is concave, and they are made of a material having a thermal expansion coefficient of the same order or larger than that of the deposited silicon nitride. The silicon nitride layer is deposited by bringing the relevant wall portions in contact at higher temperature with a flowing gas mixture containing silicon in a volatile compound and furthermore containing hydrogen.

nited States Patent [191 Asselman et al.

[ Dec. 3, 1974 [22] Filed:

[ HOT-GAS ENGINE AND METHOD 0 MANUFACTURING SAME [73] Assignee: U.S. Philips Corporation, New

' York, NY.

June 21, 1973 [21] 1 Appl. No.: 372,282

[30] Foreign Application Priority Data 8/1965 Kuntz ll7/DIG. l2 9/1973 Neelen 60/517 OTHER PUBLICATIONS R. A. Whitner, Apparatus for the Deposition of Silicon Nitride, Technical DIG. No. 11, July 1968, pgs. 5 & 6.

Primary ExaminerEdgar W. Geoghegan Assistant Examiner-H. Burks, Sr. Attorney, Agent, or FirmFrank R. Trifari 5 7 ABSTRACT A hot-gas engine, containing a working medium which consists mainly hydrogen. The walls of the working spaces of the engine which are at a higher temperature during operation are covered with a silicon nitride layer. The shape of the relevant wall portions is concave, and they are made of a material having a thermal expansion coefficient of the same order or larger than that of the deposited silicon nitride. The silicon nitride layer is deposited by bringing the relevant wall portions in contact at higher temperature with a flowing gas mixture containing silicon in a volatile compound and furthermore containing hydrogen.

' 14 Claims, 1 Drawing Figure HOT-GAS ENGINE AND METHOD OF MANUFACTURING SAME BACKGROUND OF THE INVENTION The invention relates to a hot-gas engine comprising at least one variably volume space of higher mean temperature which communicates with at least one variable volume space of lower mean temperature, each of the connections between said spaces incorporating a heater, a regenerator and a cooler; said spaces and the connections therebetween are filled with a working medium which consists mainly of hydrogen and which can flowto and fro through the regenerator.

Hot-gas engines of the kind set forth are known. In such type of engine the working medium completes a cycle such that this medium is compressed under the influence of a piston when it is situated mainly in a space of lower mean temperature. Subsequently, the medium flows, via cooler, regenerator and heater, while taking up heat in the latter two, to a space of higher mean temperature where the working medium expands.

Air or helium can be used as the working medium, but particularly the use of hydrogen is very advanta- 'geous because of its low flow losses. Even though this was known, hydrogen was not used thus far because hydrogen very quickly diffuses through the construction materials available for the walls of the engine at higher temperatures. As a result, the engine power is reduced and the escaped hydrogen must be replenished. This implies very frequent engine maintenance, which is not acceptable in practice.

SUMMARY OFTHE NEW INVENTION reach a high temperature during operation, are provided on their inner side with a layer of silicon nitride; the relevant wall portions have a concave shape on their inner side and are made of a material having a ment being necessary.

thermal expansion coefficient on-the same order or of a higher order than that of silicon nitride. The silicon nitride layer is deposited by bringing the relevant wall portions in contact at higher temperature with a flowing gas mixture which contains silicon in a volatile compound and which further contains for example hydrogen.

It was found that a silicon nitride layer which is deposited at higher temperature from the gaseous phase on a concave surface of a material having an expansion coefficient which is approximately equal to or larger than that of silicon nitride, constitutes a very good barrier against hydrogen diffusion through the relevant wall portions. As a result of the choice of the special shape, the expansion, the expansion coefficient and the deposition temperature, it is achieved that the silicon nitride layer will have no cracks or substantially none,

not even after prolonged periods of operation involving high and varying temperatures and pressures. This is probably due'to the fact that the silicon nitride layer is substantially always subjected to compressive stress. It

is thus possible to use hydrogen as the working medium From the semiconductor technique it is known that silicon nitride layers, deposited on a silicon substrate by pyrolysis from a gas mixture containing silicon in a volatile pound and ammonia, are watertight. However, no steps are taken to ensure that such a layer still counteracts hydrogen diffusion after having been subjected to high and changing temperatures and pressures and after plastic deformation for a prolonged period of time.

In order to ensure that the occurence of tensile stresses in the silicon nitride layer is precluded during operation, a-further embodiment of the hot-gas engine according to the invention is characterized in that the silicon nitride layer is deposited at a temperature which is higher than 750 C and which does not differ by more than C from the maximum temperature occuring during operation. The deposition of a layer should generally be effected at temperatures of between 750 C and 900 C, an amorphous and properly gastight layer then being obtained. At lower temperatures, the growth rate of the layer is too small, while beyond 900 C the layer becomes increasingly crystalline, which does not benefit the gastightness. The layer thickness usually lies between 0.1 pm and l urn.

The invention furthermore relates to a method of manufacturing a hot-gas engine which is provided witha heater which is composed of a number of pipes which are connected by soldering to the generator on the one side and on the otherside to the space of higher mean temperature.

7 The method is characterized in that the pipes are soldered to the regenerator and the space of higher temperature in a soldering oven, after which the assembly is cooled, the pipes being rinsed through with a gas mixture containing silicon in a volatile compound and furthermore containing forexample hydrogenia when a temperature of 900 C is reached, a layer of silicon nitride then being formed on the inner wall of the pipes.

Another method of manufacturing a hot-gas engine according to the invention is characterized in that the hot-gas engine is filled, after assembly, with agas mixture which contains silicon in a volatile compound and which furthermore contains for example hydrogen, after which the temperature of the engine is raised to the value required for the deposition of the layer by means of its own heating system, the engine being subsequently started and the gas mixture being guided along the hot portions, a layer of silicon nitride then being deposited on said portions.

The invention will be described in detail with reference to the drawing which is a diagram representation of a hot-gas engine and is not to scale.

DESCRIPTION OF TI-IE PREFERRED EMBODIMENT The reference 1 denotes a cylinder in which a piston -2 and a displacer 3 are arranged to be movable. The

The heater 9 consists of a ring of pipes, one end of the pipes 1 l of which communicating with the regenerator 8 and their other end communicating with a ring duct 12, the pipes 13 connecting the ring duct 12 to the expansion space 10. Arranged about the heater 9 is a pre-heater 14 in which air and compound gases are heat exchange. This pre-heated air is applied to a burner 15 which receives fuel via a duct 1 it is assumed that the operation of an engine of this kind is known. Hydrogen is present as the working medium in the space inside the engine. The pipes constituting the heater 9 are provided on their inner side with a layer of silicon nitride. This silicon nitride can be deposited on the pipes, for example, by heating the pipes prior to mounting in an isothermal oven to a tempera ture of between 750 C and 900C, and by subsequently feeding a gas mixture, containing silicon in a volatile compound and furthermore containing for example ammonia, through these pipes.

Heater pipes which are made of Multimet expansion coefficient 18.1-/C on the average from 20-1000C) and having a inner diameter of 3mm and a length of 700 mm can thus be heated to 840 C in an isothermal oven. Through the pipe a mixture of 5% silane (SiH.,) in argon, ammonia (NH and hydrogen ratio in volume parts of equal pressure and temperature 0.05 1) 1 :50 is fed at a rate of 2.6 litres per minute. in minutes a homogeneous layer having a thickness of 0.12 pm was thus obtained. The hydrogen diffusion coefficient of this pipe amounted to 0.15 cm mmldm hour, atm., at 750 C and 30 atm. After bending the straight pipe so as to form a bend having a radius R of 52 mm, the hydrogen diffusion coefficient amounted to 0.55 at 750C and 30 atm. It is to be noted that the hydrogen diffusion coefficient of the uncoated pipe amounted to 26 at 750 C and 30 atm. This demonstrates that the deposited silicon nitride layer very well counteracts the hydrogen diffusion, also after major plastic deformation.

Another possibility of depositing the silicon nitride layer is to introduce a gas mixture of the described kind as the working medium, after which the temperature of the heater is raised to the value desired for deposition of the layer, by means of the burner, and the engine is started. The gas mixture will then flow to and fro, during which a layer of silicon nitride will be deposited on the hot portions. The thickness of the deposited layer lies in the order of some tenths of micrometres. Deposition takes place at temperatures of between 750C and 900C because beyond this value the layer becomes increasingly crystalline with the result that the hydrogen diffusion starts to increase again. 1n the said temperature range the layer formed is amorphous and has a thermal expansion coefficient in the order of 4 X 10/C between 0 C and 1,000 C; in combination with the concave shape of the inner surface of the pipes and the expansion coefficient of the material of the pipe which is larger than that of the deposited silicon nitride, a layer is then formed which very effectively prevents hydrogen diffusion also after prolonged operation. Some of the materials which are suitable for the pipes are already said Multimet and lnconel, Hayenes 25 and Hastelloy. The composition and expansion coefficient thereof are:

Multimet: Co 18.5; Ni 19; Cr 20; Mn 1; Ta 0.75; W

2; Si 1, M0 2.5; Fe remainder; expansion coefficient 18.1 X 10"( 20-1,000 C) 4 lnconel Ni 73; Cr 15; Fe 7; Mn 0.7; Ti 2.5; Si 0.3; A1 0.9; Nb 0.9; expansion coefficient 17 X 10 (20900 C).

Haynes 25: Co 52; Ni 10; Cr 20; Fe 3; W 15; expansion coefficient 16.5 X 10""'( 20900 0 Hastelloy: Co 1.5; Ni remainder; Cr 22; Fe 18.5; Mo

9; expansion coefficient 16.5 X 10 0l,000 C).

It will be obvious that the deposited silicon nitride layer enables the use of hydrogen as the working medium in a hot-gas engine, thus offering all relevant advantages without frequent maintenance being necessary.

What is claimed is:

l. A hot-gas engine comprising one or more spaces of varying volume and higher mean temperature and one or more spaces of varying volume and lower mean temperature which are connected thereto, each of the connections between said spaces incorporating a regenerator, the said spaces and the connections therebetween being filled with a working medium which consists mainly of hydrogen and which flows to and fro through the regenerator, characterized in that the walls of the said spaces which are at a high temperature during operation and the walls of the connections are covered with a layer of silicon nitride, the relevant wall portions having a concave shape and being made of a material having a thermal expansion coefficient which is of the same order or larger than that of the deposited silicon nitride, the silicon nitride layer being deposited by bringing the relevant wall portions in contact at higher temperatures with a flowing gas mixture which contains silicon in a volatile compound and which furthermore contains hydrogen.

2. A hot-gas engine as claimed in claim 1, characterized in that the layer of silicon nitride is deposited at a temperature of the relevant wall portions which is higher than 750C and which does not differ by more than C from the maximum temperature occuring in this wall during operation.

3. A method of manufacturing a hot-gas engine as claimed in claim 1, the said engine being provided with a heater which is composed of a number of pipes, one end of which is connected by soldering to the regenerator and the other end of which is connected by soldering to the space of higher mean temperature, characterized in that the pipes are soldered to the regenerator and the space of higher temperatures in a soldering oven, after which the assembly is cooled, the pipes being rinsed through, when a temperature of 900C is reached, with a gas mixture which contains silicon in a volatile compound and which furthermore contains inter alia hydrogen, a layer of silicon nitride then being formed on the inner wall of the pipes.

4. A method of manufacturing a hot-gas engine as claimed in claim 1, characterized in that the hot-gas engine, after having been assembled, is filled with a gas mixture which contains silicon in a volatile compound and hydrogen, after which the temperature of the engine is raised, by means of its heating system, to the value which is desired for the deposition of the layer, after which the engine is started and the gas mixture is fed along the hot portions, a layer of silicon nitride then being deposited on said portions.

5. In a hot gas engine operable with a working medium and including variable volume compression and expansion chambers operable at respectively higher and lower mean temperatures, said chambers having walls with inner surfaces that define the compression and expansion spaces therein, connection means including a regenerator through which said chambers communicate, and a heater formed of pipes the walls of which have inner surfaces which define heater space which communicates with said expansion space, the improvement in combination therewith of a layer of silicon nitride on said inner surfaces of said heater pipes, said heater walls comprising material having a thermal expansion coefficient which is at least as great as the thermal coefficient of the silicon nitride on said walls.

6. Apparatus according to claim 5, wherein said layer of silicon nitride is also on the inner surfaces of said expansion chamber walls.

7. Apparatus according to claim 5, wherein said wall surfaces having silicon nitride thereon are substantially concave.

8. Apparatus according to claim 7, wherein said heater pipes are essentially round, with the inner surfaces of the pipe walls being concave.

9. In a hot gas engine operable with a working medium and including variable volume compression and expansion chambers operable at respectively higher and lower mean temperatures, said chambers having walls with inner surfaces that define the compression and expansion spaces therein, connection means including a regenerator through which said chambers communicate, and a heater formed of pipes the walls of which have inner surfaces which define heater space which communicates with said expansion space, wherein said expansion chamber walls and heater pipe walls are heated to high temperatures during operation of the engine, the improvement in combination there with comprising a layer of silicon nitride on the inner surfaces of walls of at least one of said heater pipe and expansion chamber elements.

10. Apparatus according to claim 5 wherein said layer has thickness in the range of 0.1 pm to 1.0 ,um.

11. Apparatus according to claim 5 wherein said heater pipe material comprises one of the materials selected from the group consisting of Multimet, Inconel, Haynes 25, and Hastelloy.

12. In a method of manufacturing a hot gas engine as described in claim 7 the improvement in combination therewith of preparing the inner surfaces of the heater pipe walls to resist diffusion of hydrogen gas therethrough comprising the steps, joining the heater pipes to the regenerator and expansion chamber by soldering, cooling said soldered elements to a temperature in the range of 750C to 900C, rinsing said heater pipe inner surfaces with a gas mixture containing silicon in a volatile compound including hydrogen, and thereby forming a layer of silicon nitride on said surfaces.

13. A method according to claim 12 comprising the further steps of forming a layer of said silicon nitride on the inner surfaces of said expansion chamber as such layer is formed on said heater pipe surfaces.

14. in a method of manufacturing a hot gas engine as described in claim 7 and particularly as regards an engine after same is assembled, for preparing the inner surfaces of the heater pipe walls thereof to resist diffusion of hydrogen gas therethrough, comprising the steps (a) filling the spaces in said expansion chamber and heater pipes with a gas mixture containing silicon in a volatile compound and hydrogen, (b) raising the engine temperature to a temperature in the range of 750C to 900C, (0) starting and operating the engine whereby said gas mixture flows in contact with said inner surfaces and said layer of silicon nitride is formed thereon. 

1. A hot-gas engine comprising one or more spaces of varying volume and higher mean temperature and one or more spaces of varying volume and lower mean temperature which are connected thereto, each of the connections between said spaces incorporating a regenerator, the said spaces and the connections therebetween being filled with a working medium which consists mainly of hydrogen and which flows to and fro through the regenerator, characterized in that the walls of the said spaces which are at a high temperature during operation and the walls of the connections are covered with a layer of silicon nitride, the relevant wall portions having a concave shape and being made of a material having a thermal expansion coefficient which is of the same order or larger than that of the deposited silicon nitride, the silicon nitride layer being deposited by bringing the relevant wall portions in contact at higher temperatures with a flowing gas mixture which contains silicon in a volatile compound and which furthermore contains hydrogen.
 2. A hot-gas engine as claimed in claim 1, characterized in that the layer of silicon nitride is deposited at a temperature of the relevant wall portions which is higher than 750*C and which does not differ by more than 100*C from the maximum temperature occuring in this wall during operation.
 3. A method of manufacturing a hot-gas engine as claimed in claim 1, the said engine being provided with a heater which is composed of a number of pipes, one end of which is connected by soldering to the regenerator and the other end of which is connected by soldering to the space of higher mean temperature, characterized in that the pipes are soldered to the regenerator and the space of higher temperatures in a soldering oven, after which the assembly is cooled, the pipes being rinsed through, when a temperature of 900*C is reached, with a gas mixture which contains silicon in a volatile compound and which furthermore contains inter alia hydrogen, a layer of silicon nitride then being formed on the inner wall of the pipes.
 4. A method of manufacturing a hot-gas engine as claimed in claim 1, characterized in that the hot-gas engine, after having been assembled, is filled with a gas mixture which contains silicon in a volatile compound and hydrogen, after which the temperature of the engine is raised, by means of its heating system, to the value which is desired for the deposition of the layer, after which the engine is started and the gas mixture is fed along the hot portions, a layer of silicon nitride then being deposited on said portions.
 5. In a hot gas engine operable with a working medium and including variable volume compression and expansion chambers operable at respectively higher and lower mean temperatures, said chambers having walls with inner surfaces that define the compression and expansion spaces therein, connection means including a regenerator through which said chambers communicate, and a heater formed of pipes the walls of which have inner surfaces which define heater space which communicates with said expansion space, the improvement in combination therewith of a layer of silicon nitride on said inner surfacEs of said heater pipes, said heater walls comprising material having a thermal expansion coefficient which is at least as great as the thermal coefficient of the silicon nitride on said walls.
 6. Apparatus according to claim 5, wherein said layer of silicon nitride is also on the inner surfaces of said expansion chamber walls.
 7. Apparatus according to claim 5, wherein said wall surfaces having silicon nitride thereon are substantially concave.
 8. Apparatus according to claim 7, wherein said heater pipes are essentially round, with the inner surfaces of the pipe walls being concave.
 9. In a hot gas engine operable with a working medium and including variable volume compression and expansion chambers operable at respectively higher and lower mean temperatures, said chambers having walls with inner surfaces that define the compression and expansion spaces therein, connection means including a regenerator through which said chambers communicate, and a heater formed of pipes the walls of which have inner surfaces which define heater space which communicates with said expansion space, wherein said expansion chamber walls and heater pipe walls are heated to high temperatures during operation of the engine, the improvement in combination therewith comprising a layer of silicon nitride on the inner surfaces of walls of at least one of said heater pipe and expansion chamber elements.
 10. Apparatus according to claim 5 wherein said layer has thickness in the range of 0.1 Mu m to 1.0 Mu m.
 11. Apparatus according to claim 5 wherein said heater pipe material comprises one of the materials selected from the group consisting of Multimet, Inconel, Haynes 25, and Hastelloy.
 12. In a method of manufacturing a hot gas engine as described in claim 7 the improvement in combination therewith of preparing the inner surfaces of the heater pipe walls to resist diffusion of hydrogen gas therethrough comprising the steps, joining the heater pipes to the regenerator and expansion chamber by soldering, cooling said soldered elements to a temperature in the range of 750*C to 900*C, rinsing said heater pipe inner surfaces with a gas mixture containing silicon in a volatile compound including hydrogen, and thereby forming a layer of silicon nitride on said surfaces.
 13. A method according to claim 12 comprising the further steps of forming a layer of said silicon nitride on the inner surfaces of said expansion chamber as such layer is formed on said heater pipe surfaces.
 14. In a method of manufacturing a hot gas engine as described in claim 7 and particularly as regards an engine after same is assembled, for preparing the inner surfaces of the heater pipe walls thereof to resist diffusion of hydrogen gas therethrough, comprising the steps (a) filling the spaces in said expansion chamber and heater pipes with a gas mixture containing silicon in a volatile compound and hydrogen, (b) raising the engine temperature to a temperature in the range of 750*C to 900*C, (c) starting and operating the engine whereby said gas mixture flows in contact with said inner surfaces and said layer of silicon nitride is formed thereon. 